1. Field of the Invention
The invention relates to annular friction and seal devices to be placed around the impeller blades of a gas turbine or high pressure axial compressor stage. For example, in a stator-and-rotor machine, around the impeller blade system defining the hot gas stream. More specifically, it concerns such a device in association with means for cooling the friction seal and protecting the stator from the heat of the gases in the stream. The device is of the type which comprises successively from periphery to center: a support-ring surrounding the impeller, a first annular, air-permeable layer of material called the "cooling layer" fastened to the support ring, a second annular layer of material called the "friction layer" which is joined to the cooling layer, lies in immediate proximity to the ends of the impeller blades, and can be abraded by these blade ends, and a system for bringing cool air into the cooling layer.
2. Description of the Prior Art
The friction layer usually consists of a porous material (aggregate, felt, foam, perforated plate, etc.) that can be abraded by the ends of the blades. Unless some special arrangement is made, the cooling air passes through this layer and flows at least in part toward the hot gas stream.
The U.S. Pat. No. 4,392,656 Tirole, et al describes a device of this type in which the two annular layers are separated by a middle layer, the permeability of which is such that the radial flow of air which crosses the middle layer is appreciably less than the flow of axial air that crosses the cooling layer.
The middle layer may even be impermeable to air so that the air flows only axially (i.e., in a direction parallel to the axis of the machine) in the cooling layer. All direct interaction between the two layers is eliminated. The friction layer is cooled by thermal conduction toward the cooling layer. This arrangement, intended to keep to seal function of the friction layer independent from the cooling function of the cooling layer (which also protects the support-ring from the heat of the gas stream), makes it possilbe to eliminate the disadvantages inherent in cooled annular friction seal means that are traversed by all or a large part of the flow of cooling air. These disadvantages include the necessity of introducing the cooling air at a pressure appreciably higher than the pressure in the gas stream, the existence within the cooling layer of an axial flow that more or less opposes the radial flow, and the gradual loss of cooling efficiency as the friction zone grows less permeable, which occurs as a result of contamination of the layer by the gases in the stream and "pasting up" of the pores in the surface in contact with the ends of the blade.